1. Field of the Invention
The present invention relates to replay optics for holographic displays and in particular to replay optics for generating a three dimensional image from an illuminated spatial light modulator.
2. Discussion of Prior Art
It is well known that a three-dimensional image may be presented by forming an interference pattern or hologram on a planer surface. The three-dimensional image is visible when the hologram is appropriately illuminated. Recently, interest has grown in so-called computer generated holograms (CGHs) which offer the possibility of displaying high quality images, which need not be based upon real objects, with appropriate depth cues and without the need for viewing goggles. Interest is perhaps most intense in the medical and design fields where the need for realistic visualization techniques is great.
Typically, a computer generated hologram involves the generation of a matrix of data values (each data value corresponding to a light transmission level) which simulates the hologram which, might otherwise be formed on a real planer surface. The matrix is applied to a Spatial Light Modulator (SLM) which may be, for example, a two-dimensional array of liquid crystal elements or of acousto-optic modulators. Coherent light is directed onto the SLM using for example a laser such that the resulting output, either reflected from the SLM or transmitted through the SLM, is a modulated light pattern. An example of an SLM is an Electrically Addressable SLM (EASLM).
In order to produce a three-dimensional image of usable size and viewing angle, the SLM typically has to have a large number of pixels, e.g. 1010. In addition, the pixels of the SLM must be positioned relative to one another with a high degree of accuracy. The device must also be capable of modulating coherent light, e.g. produced by a laser. These requirements are extremely demanding and expensive to achieve in practice.
One approach is presented in GB2330471A and is illustrated schematically in FIG. 1. This document describes a holographic display technique, which is referred to as Active Tiling™, and involves the use of a relatively small EASLM 1 in combination with a relatively large Optically Addressable Spatial Light Modulator (OASLM) 2. This part of the system comprises the so-called “replicating” optics. The holographic matrix is subdivided into a set of sub-holograms, with the data for each sub-hologram being passed in turn to the EASLM 1. The EASLM 1 is illuminated from one side with incoherent light 3. The OASLM 2 comprises a sheet of bistable liquid crystal (in one example the liquid crystal is a ferroelectric liquid crystal) which is switched from a first to a second state by incident light. Replicating optics 4, disposed between the EASLM 1 and the OASLM 2, cause the output of the EASLM 1 (i.e. light transmitted through the EASLM 1) to be stepped across the rear surface of the OASLM 2. The bistable nature of the OASLM liquid crystal means that the portion or “tile” of the OASLM 2 onto which a sub-holographic image is projected, remembers that image until such time as the OASLM is reset by the application of an electrical voltage. It will be appreciated that, providing a reset voltage is applied only at the end of a complete scan, immediately prior to reset the OASLM 2 will have “stored” in it a replica of the complete holographic matrix. An alternative arrangement which avoids the need for an OASLM by making use of the “memory” of a human eye is described in PCT/GB00/01903.
Considering the arrangement of FIG. 1, the CGH displayed on the OASLM 2 is “read” using an arrangement such as that illustrated in FIG. 2. This arrangement is referred to as the “replay” optics. The OASLM 2 is typically illuminated with a plane wave originating from a point source 15 and the reflected light is focused down by replay optics 7,8 to form a 3D image. Normal incidence illumination is achieved with the use of a beam splitter 5, although a slight off axis angle of illumination may alternatively be used. In either case, large collimating optics 6 are required to provide a large illumination wavefront. The need for a beam splitter 5 and collimating optics 6 adds to the cost of the system as high quality, large aperture (large FOV) optics are expensive. To reduce costs, lower quality optics may be used. However this will result in poorer performance.